Electrical prod structure



May 19, 1970 R. sTRAcHAN ELECTRICAL PROD STRUCTURE 2 Sheets-Sheet 1 Filed March 15, 1967 May19,197o- RSTRACHAN 3,513,388 I ELECTRICAL PROD STRUCTURE Filed March 1s. 1967 2 sheets-Shen 2 United States Patent O ELECTRICAL PROD STRUCTURE Ronald Strachan, Sycamore, Ill., assigner to Ideal Industries, Inc., Sycamore, Ill., a corporation of Delaware Filed Mar. 13, 1967, Ser. No. 622,547 Int. Cl. G01r 1/06 U.S. Cl. S24-72.5 1 Claim ABSTRACT F THE DISCLOSURE An electric contact prod and prod retaining structure. The contact prod has a hollow handle housing a current limiting device which is connected at one end to a lead wire by a collar, having a flange which engages a shoulder formed in the handle, and at the other end to a metal spike; the hollow handle also has stepped Surfaces at the end from which the lead wire exits the handle so that the wire will randomly flex at more than one point along its length when the prod is manipulated thereby reducing the probability of fatigue failure of the wire. The prod retaining structure has a receiving cavity formed with an upwardly extending tongue or a deflecting ledge to cause the metal spike to be deflected upon inserting the contact prod into the cavity thereby ellecting light binding action between the prod handle and the cavity and removably retaining the prod within the retaining structure.

This invention resides in the eld of electric contact prod structures and is an improvement on prior such structures used to establish contact between any type of electric testing device and a source of electric current. Such devices are generally composed of a meter or instrument in a housing with two or more flexible leads extending therefrom having manually manipulatable prod structures on the ends thereof, with each prod structure having an exposed metal spike for making electrical contact with leads, sockets, and the like.

One of the primary objects of this invention is to provide a method of minimizing failure in the flexible lead due to llexure at the exit point of such lead from the contact prod handle.

Another object is to provide a strain relieved joint between the flexible lead wire and the current limiting device in a current limited contact prod structure.

Another object is to provide a contact prod handle which contains slots of different effective depths at the exit end of the flexible wire lead so that during normal use, such wire lead randomly ilexes at a plurality of points.

Another object is to provide a compact method of mechanically securing and electrically connecting the flexible wire lead to a current limiting device and at the same time providing stress relief to such current limiting device.

Another object is to provide a test prod retaining structure into which a test prod may be removably inserted.

Another object is a structure which will house and retain a test prod while facilitating removal rof such test prod from such housing structure.

Another object is to provide a simple and economical means of mounting and removably retaining a test prod.

Other objects will appear from time to time in the ensuing specification, claims and drawings in which:

FIG. 1 is a plan view, in partial cross-section, of the contact prod disclosed herein;

FIG. 2 is a sectional view along line 2-2 of FIG. l;

FIG. 3 is a sectional View along line 3 3 of FIG. l;

FIG. 4 is an end view of FIG. 1;

IFIG. 5 is a View in partial cross-section showing one form of the contact prod mounting and retaining means of this invention;

FIG. 6 is a view in partial cross-section showing a variation of the contact prod mounting and retaining means of this invention; and

FIG. 7 is a sectional view along line 7-7 of FIG. 6.

In FIG. l, the electric contact prod has been shown as consisting of a tubular prod handle 10, which is molded or otherwise formed using a substantially rigid insulating plastic such as that sold under the trademark Lexan, or the like; an insertible plastic spike holder 12 containing a metal spike 14; a flexible wire lead 16 which enters the hollow -prod handle 10 at one end and is terminated within such handle at joint 18; and an axial lead current limiting device 20 having lead ends 22 and 24, with one such end 22 mechanically and electrically connected to the llexible wire lead 16 at joint 18, and the other lead end 24 mechanically fixed and electrically joined to the metal spike 14.

The hollow contact prod handle 10 contains a series of tapered bores 26, 28 and 30 passing centrally throughout its length. Each succeeding tapered bore is formed larger than the preceding bore thereby forming shoulders 32 and 34 between such successive tapered bores. Tapered vbore 26 is the smallest of such bores, having an inside diameter slightly larger than the outside diameter of the flexible wire lead 16. Tapered bore 28 is larger than bore 26, having a diameter large enough to house the current limiting device 20.

At the flexible wire lead entry end of the handle 10, two diametrically opposed slots 35 and 36 'have been formed in the handle outside wall, with such slots running parallel to the center line of such handle and having different effective depths. One slot 35 has parallel side walls and a radial end whereas slot 36 is tapered. At the metal spike end, the handle is generally shaped to engage with and retain the plastic spike holder 12. The handle 10 may have any outside contour which facilitates handling of the contact prod.

As shown in FIG. 2, the electrical joint, indicated generally at 18, between the llexible wire lead 16 and the current limiting device 20, consists of a hollow flanged collar 37 having a hole 38 which is sized to accept the stripped end of the flexible wire lead 16 and the axial lead 22 of the current limiting device 20. The outside diameter of the collar llange 39 is approximately equal to but slightly smaller than the diameter of tapered bore 28 at its junction with the shoulder 32, so that the collar 37 may be inserted within tapered bore 28 and seat on shoulder 32. The collar 37 is made using a material having physical properties such that when the stripped end of the llexible wire lead 16 and t-he axial lead 22 of the current limiting device 20 are soldered therein, the electrical connection and flange 39 are stronger than the wire lead 16 so that the exible wire lead 16 will break before the electrical connection 18 or collar 37 are damaged when the flexible lead 16 is subjected to an axial pull. As shown in FIGURE 2, the wire lead 24 of the current limiting device 20 is electrically and mechanically joined to the metal spike 14, thereby completing the circuit between the metal spike and t-he flexible wire lead 16.

Although the electrical contact prod has been shown as having a joint 18 connecting the flexible wire lead to a current limiting device, it should be undertsood that the contact prod may be constructed without such electrical joint 18 and current limiting device 20, so that the flexible lead 16 is electrically and mechanically joined directly to the metal spike 14.

In FIG. 5 the contact prod is shown as having a springy metal spike 50 which is retained by prod handle 52. The

3 Contact prod is shown inserted in a receiving cavity 54 formed in a retaining structure 56. The receiving cavity is formed with a sloping ledge 58 which is positioned so that, when the contact prod is inserted into the receiving cavity, the springy metal spike 50 engages the ledge 58 and is deflected thereby. The receiving cavity above the deflecting ledge 58 is formed so that the deflection of the springy metal spike 50 causes two enlarged areas of the prod handle, such as at 60 and 62 to press against the inner surface o-f the cavity 54, as at 64 and 66. A protruding flange 68 is provided which is substantially larger in cross-section than the opening of the cavity 54 so that suc-h flange `will engage the top surface 70 of the test prod retaining structure 56 thereby limiting the depth to which the contact prod may be inserted into the receiving cavity.

Although the deflecting means has been shown as a ledge 58 against which the springy metal spike 50 deflects, it should be understood that other configurations could be used to ac-hieve the same purpose. For example, as shown in FIGS. 6 and 7, a tongue 72 may extend upward into the receiving cavity 54, thereby deflecting the metal spike 50 randomly to one side or the other, either way sufficing to retain the contact prod in the cavity 54.

The use, operation and function of this invention are as follows:

By repeatedly flexing a wire lead at a specific point along its length, the wire lead may be broken at such flexure point due to fatigue. During the normal use of an electrical contact prod, the flexible wire lead connecting such prod to the monitoring device receives considerable flexing at the point of entry into the 4contact prod handle. Accordingly, the flexible wire lead is highly susceptible to failure at such entry point. It has been found that the susceptibility of the flexible wire lead to fatigue failure due to flexure is greatly minimized if the contact prod handle rwire lead entry end is constructed so that the Wire lead flexes randomly at various points along its length during normal use of the contact prod.

Therefore, this invention provides two axial slots 35 and 36 in the wall of the contact prod handle 10` at the entry end of the flexible wire lead 16. As the test prod is manipulated during normal use, the flexible wire lead 16 may randomly enter either slot. Each slot has been constructed to have a different effective depth. As shown by FIGURES 2 and 3, one slot has parallel side walls whereas the other slot is tapered. The entry end of the contact prod handle thus provides three surfaces over which the flexible wire lead 16 may flex during normal manipulation of the contact prod. Accordingly, the wire lead may flex over the end 33 of the contact prod handle 10 as shown in FIG. 2, or the surface formed by the bottom of the slot 35 as shown in FIG. 1, or the surface formed by the tapered slot 36 as shown in FIG. 3. With the -prod handle construction of this invention, during normal manipulation of the electrical contact prod, it is probable that such wire lead will bend over and thereby flex an equal number of times at each of the three staggered flexing surfaces, as shown in FIGS. 1-3 respectively, thereby eliminating flexure of the Wire at only one point along its length and minimizing the probability of failure of such Wire due to ilexure fatigue. Although the contact prod handle has been depicted and described as having two slots and therefore a three-stepped structure at the flexible wire lead entry end, it should be understood that such handle may contain more or less than three steps and such steps may be formed other than by slotting.

In many electrical contact prod applications, it is desirable to have a current limiting device, such as a resistor, interposed in the electrical circuit between the metal spike and the flexible wire lead connecting suc-h spike to the monitoring equipment. Since the flexible wire lead may be subjected to axial tugging or pulling during normal use of the contact prod, which, if transmitted directly to the current limiting device would be harmful to such device, it is further desirable to insulate the current limiting device from such harmful forces.

The current limiting device v20 is electrically connected to the flexible wire lead 16 at the joint 18 with such joint also protecting such current limiting device from harmful axial forces applied to the flexible wire lead 16. In constructing the strain relieving electrical joint 18, the flexible wire lead 16 is passed through the hollow prod handle 10 so that the stripped end of such wire extends out of the metal spike end of the handle 10. The stripped end of the flexible wire lead 16 and one lead 22 of the current limiting device 20 are inserted from opposite sides into the hollow flanged collar 37 with the current limiting device lead 22 being inserted from the flanged end. The leads are soldered to each other and the collar 37 so as to create a sound mechanical and electrical connection between such elements. The other lead 24 of the current limiting device 20 is electrically and mechanically attached to the metal spike 14. The wire lead 16 is then withdrawn into the handle 10 until the collar flange 39 abuts the shoulder 32 and the plastic spike holder 12 is engaged with the handle 10. With the joint 18 of this invention, an axial force or pull on the flexible wire lead 16 is transmitted to the collar 37 and through the collar flange 39 to the prod handle shoulder 32, thereby insulating the current limiting device 20 from such forces.

An economical means of housing and removably retaining a contact prod is disclosed by this invention wherein a test prod, having a springy metal spike 50, a flange 68, and two enlarged areas 64 and 66 along the contact prod handle, is removably inserted and retained into and by a receiving cavity 54. As the contact prod is inserted into the cavity 54, the springy metal spike 501 engages deflection means such as the ledge 58 or tongue 75 which deflects the spike, thereby causing the contact prod handle to press against the inner surface of the cavity 54 at two enlarged areas, `64 and `66, of the contact prod handle. The contact prod is thus retained by static friction existing at three pressure points created by the springy metal spike and the two enlarged handle areas 60 and 62 pressing against the inside surface of the receiving cavity 54.

The depth to which the contact prod may be inserted into cavity 54 is regulated by the protruding flange 68 which engages the receiving structure top surface 70. Since the degree to which the springy metal spike is deflected depends upon the depth of insertion of the contact prod into the receiving cavity and upon the length of the deflecting ledge 58 or height of the deflecting tongue 72, and since the amount of static friction existing at the three pressure points is dependent upon the amount of springy metal spike deflection, the amount of force required to insert or remove the contact prod into or from the receiving cavity 54 may be regulated by the location of flange 68 and by controlling the length or llieight of the deflecting ledge 58 or tongue 72 respective y.

While the preferred form and variation of the invention have been described, it should be understood that suitable other additions, alterations, and variations may be made without departing from the inventions fundamental theme.

What I claim is:

1. In a contact prod structure including an elongated generally hollow handle otherwise open at both ends with a bore in between, an insulated lead wire extending into the bore of the handle at one end and adapted to be flexed relative thereto when the prod structure is manipulated in use, a metal contact spike projecting from the other end of the bore so as to be capable of direct metal-to-metal contact when the prod structure is in use, a connection between the inner end of the lead wire and the contact spike to complete the contact between the exposed end of the metal spike and the lead wire, the open end of the bore into which the lead wire extends being structured so as to minimize fatigue and failure of the insulation on the lead wire and the wire itself when the prod is in use, such structure including a plurality of stepped surfaces in the form of and including at least two slots elongated somewhat in an axial direction and spaced from each other circumferentially around the handle, each having a circumferential dimension relative to the diameter of the lead Wire such that the wire may flex therethrough so as to project, in eiect, through the side of the handle, the inner portion of one slot being shaped to cause flexing of the lead wire at a different point along the wires length than the point at which the lead wire is flexed by the inner portion of the other slot, the open end of the handle having peripheral surfaces between the slots of a circumferential extent sucient to cause the lead wire to Hex thereover so that the lead wire, in use, will be flexed in at least three points along the axial length of the wire when in use.

ALFRED E.

References Cited UNITED STATES PATENTS SMITH,

Primary Examiner U.s. C1. X.R. l5 324-149; 174-46, 135 

